Bond length
Bond length or bond distance in
molecular geometry is the distance between two
bonded atoms in a
molecule.
Bond length is directly related to
bond order, when more
electrons participate in bond formation the bond will get shorter. Bond length is also directly related to
bond strength and the
bond dissociation energy as a stronger bond is also a shorter bond. In a bond between two identical atoms half the bond distance is equal to the
covalent radius. Bond lengths are measured in molecules by means of
x-ray diffraction. A set of two atoms sharing a bond is unique going from one molecule to the next. For example the carbon to hydrogen bond in
methane is different from that in
methyl chloride. It is however possible to make generalizations when the general structure is the same.
A table with experimental
single bonds for carbon to other elements is given below. Bond lengths are given in
picometers. By approximation the bond distance between two different atoms is the sum of the individual
covalent radii (these are given in the
chemical element articles for each element). As a general trend, bond distances
decrease across the
row in the
periodic table and
increase down a
group. This trend is identical to that of the
atomic radius.
The actual bond length between two atoms in a molecule depends on such factors as the
orbital hybridisation and the electronic and steric nature of the
substituents. The carbon carbon bond length in diamond is 154 pm which is also the largest bond length that exists for ordinary carbon covalent bonds.
Unusually long bond lengths do exists. In one
tricyclobutabenzene a bond length of 160 pm is reported. The current record holder is another cyclobutabenzene with length 174 pm based on
X-ray crystallography . In this type of compounds the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°.
|
cyclobutabenzene with a bond length in red of 174 pm |
The existence of a very long C-C bond length of up to 290 pm is claimed in a dimer of two
tetracyanoethylenedianions although this concerns a 2-electron-4-center bond . This type of bonding has also been observed in dimers of neutral
phenalene dimers. The bond lengths of these so-called
pancake bonds are up to 305 pm.
Shorter than average carbon carbon bonds distances are also possible,
alkenes and
alkynes have bond lengths of respectively 133 and 120 pm due to increased
s-character of the
sigma bond.In
benzene all bonds have the same length: 139 pm. In carbon carbon single bonds increased s-character is also notable in the central bond of
diacetylene (137 pm) and that of a certain
tetrahedrane dimer (144 pm).
In
propionitrile the
cyano group withdraws electrons also resulting in a reduced bond length (144 pm). Squeezing a CC bond is also possible by application of
strain. An unusual organic compound exists called
In-Methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a
trypticene and a phenyl group. In an
in silico experiment a bond distance of 136 pm is estimated for
neopentane locked up in
fullerene . The smallest theoretical CC single bond obtained in this study is 132 pm for a hypothetical
adamantane like molecule.
In the same study it is estimated that for
ethane it takes 2.8
kJ/
mol to stretch the CC bond by 5 pm from its equilibrium value and only 3.5 kJ/mol to squeeze it by the same amount. on the other hand, stretching and squeezing by 15 pm requires 21.9 and 37.7 kJ/mol.
*
Chemical bond*
Molecule*
Utah Chemists Find Longest Carbon-Carbon Bond Press release# Handbook of Chemistry & Physics 65th Edition CRC Press ISBN 0849304652#
Naphthocyclobutenes and Benzodicyclobutadienes: Synthesis in the Solid State and Anomalies in the Bond Lengths Fumio Toda European Journal of Organic Chemistry Volume 2000, Issue 8, Date: April
2000, Pages: 1377-1386
Abstract#
Exceptionally Long (2.9 Å) C-C Bonds between [TCNE]- Ions: Two-Electron, Four-Center *-* C-C Bonding in -[TCNE]22- Angewandte Chemie International Edition Volume 40, Issue 13, Date: July 2,
2001, Pages: 2540-2545 Juan J. Novoa, Pilar Lafuente, Rico E. Del Sesto, Joel S. Miller
Abstract#
Stable (Long-Bonded) Dimers via the Quantitative Self-Association of Different Cationic, Anionic, and Uncharged -Radicals: Structures, Energetics, and Optical Transitions Jian-Ming Lü, Sergiy V. Rosokha, and Jay K. Kochi
J. Am. Chem. Soc.;
2003; 125(40) pp 12161
Abstract]#
Aromaticity on the Pancake-Bonded Dimer of Neutral Phenalenyl Radical as Studied by MS and NMR Spectroscopies and NICS Analysis Shuichi Suzuki, Yasushi Morita, Kozo Fukui, Kazunobu Sato, Daisuke Shiomi, Takeji Takui, and Kazuhiro Nakasuji
J. Am. Chem. Soc.;
2006; 128(8) pp 2530
Abstract#
Squeezing CC Bonds Deborah R. Huntley, Georgios Markopoulos, Patrick M. Donovan, Lawrence T. Scott, Roald Hoffmann
Angewandte Chemie International Edition Volume 44, Issue 46 , Pages 7549 - 7553
2005*
Bond length tutorial 
